This invention relates generally to an improved laminated plastic construction particularly suitable for incorporation into utility meters with sealed metal and plastic lens register enclosures.
Plastic has replaced glass in a variety of situations, because it can be molded easily, can provide a clear surface similar to glass and because it is more resistant to breakage. A problem with the replacement of glass by plastic in some situations is that plastic has a high water vapor and gas vapor transmission rate as compared to glass. This problem is particularly problematic when the plastic is used to enclose or protect components that will be damaged by the water or gas.
Common residential utility meters (water, gas and electric) are often mounted outside the residence so that meter readers can obtain consumption readings without entering the home. This presents a particular problem for water meters since the measured product, water, can freeze in all but very tropical climates causing frost damage to the meter and connected piping. Common practice to solve this problem is to mount the water meter in a meter box pit below the frost line where the meter can be protected from freezing.
While warm convection air currents help prevent freezing in very cold weather, in warmer weather these pits are often hotter than ambient air. These meter boxes are not sealed; thus ground water often fills the pit. As a result of these factors, the meter box is a hot, moist environment for several months of the year, especially in much of North America.
While this environment does not present any unusual problems for standard meters with sealed metal and glass lens register enclosures, in a typical modern electronic type meter, a clear plastic cover is substituted for the glass lenses. Plastic is considered superior because it permits close dimensional fitting bosses and terminal penetrations to be molded in with the parts, an arrangement not possible with glass production. A clear plastic is needed to enable meter readers to check the mechanical odometer reading against the electronic reading at established intervals of from one year to three years; it is also needed to in-test as received meters. A high impact plastic is also required because of the rough service associated with the in-ground use. A natural choice for this application has been polycarbonate. Commonly used polycarbonates are Merlon, a trademark of Mobay Chemical Company of Pittsburgh, Pa. for a polycarbonate resin, and LEXAN, a trademark of General Electric Company, Polymers Product Department of Pittsfield, Mass., for a thermoplastic carbonate-linked polymer.
Although polycarbonate has the advantages described above, a high moisture vapor transmission rate is an offsetting poor physical property. The consequence of using a polycarbonate cover is that the driving potential of high humidity and temperature eventually permits moisture to permeate the cover causing moisture saturation of the register internals and especially the electronic circuitry. Moisture on low voltage microcircuits will eventually cause catastrophic shorts and/or lead corrosion. Conformal coatings of the electronic circuitry can provide an improved measure of moisture resistance. However, a typical electronic utility meter register contains a contact type, rotating encoder mechanism which consists of a small brush wiper which makes electrical connection with ten segment commutator pads to accomplish the encoding. This required movement defeats the ability to protect all of the circuitry with moisture-resistant coatings.
The actual process and amount of water vapor transmission through a polycarbonate cover is primarily a function of three items, (1) the lens thickness (2) the lens surface area and (3) the lens material's vapor transmission rate. Since the utility meter's electronic register is tooled and the meter's register bonnet is also reasonably fixed in dimensions, the geometry of the register enclosure envelope (items (1) and (2) above) preferably should not be altered dramatically.
Assuming that the particular geometry is fixed, moisture vapor saturation of the internal volume of the electronic register can take place in three or four days in the presence of a high driving potential (high ambient temperature an pits that are wet from lawn irrigation, tidal action, and rain, for example). To adequately protect the meter's electronic circuitry, a two order of magnitude improvement in the water vapor transmission rate is needed in the cover material construction.
Accordingly, it is an object of the present invention to provide a laminated plastic construction with superior moisture barrier properties.
It is another object of the present invention to provide a cover for an electronic utility meter of a laminated plastic with superior moisture barrier properties.
A further object of the present invention is to provide a laminated moisture barrier plastic structure as a cover for an electronic utility meter.
Still another object of the present invention is to provide a laminated moisture resistant plastic cover for an electronic water meter to overcome the source of failure of these meters at the present time.
Yet another object of the present invention is to provide laminated plastic structures that exhibit improved gas barrier properties and improved resistance to hydrolysis and attack by caustic agents.